The act of folding the broad hind wings among beetles is not less significant than the arrangement of the fore wing. If we forcibly spread out the former in a beetle which has just been killed and then leave it to its own resources again, we observe the following result: According to its peculiar mode of joining, the costal vein on the fore edge approaches the mid or discoidal vein of the basal half as well as the distal half of the wing, whence arises a longitudinal fold which curves in underneath. Then the distal half snaps under like the blade of a pocket knife and lies on the plane of the costal edge of the wing, while it also draws after it the neighboring wing-area. The soft hinder-edge portion turns in simultaneously when this wing-area remains fixed to the body while the costal portion is moving towards the middle line of the body. The wing-membranes of almost all insects have, moreover, the capability of folding themselves somewhat, and this power of extending or contracting the wing-membrane at will is of great importance in flight. Yes, but how is the folded wing spread out again? The fact may be shown more simply and easily than one might suppose, and may be most plainly demonstrated even to a larger public by making an artificial wing exactly after the pattern of the natural one, in which bits of whalebone may take the place of veins and a piece of india rubber the membrane spread out between them. The reader will be patient while we just explain to him the act of unfolding of the membranous wing of the beetle. The actual impulse for this unfolding is due to the flexor muscles which pull on, and at the same time somewhat raise the vein on the costal edge. By this means the membranous fold lying directly behind the costal vein is first spread out. But since this fold is connected with the longitudinal fold of the distal end of the wing which closes like a blade, the wing-area last mentioned which is attached to the middle fold of the wing by the elastic spring-like diagonal vein becomes stretched out. The hinder rayed portion adjacent to the body is, on the other hand, simply drawn along when the wing stands off from the body. In order to properly grasp the mechanism of the insect wing we must again examine its mode of articulation to the body somewhat more accurately. [Illustration: FIG. 171.—Longitudinal section through a Tipula: _a_, mouth; _an_, antenna; _k_{3}_, maxillary palpus; _ol_, labrum; _oG_, brain; _uG_, subœsophageal ganglion; _BG_, thoracic ganglion; _schl_, œsophagus; _mD_, digestive canal; _Ov_, ovary; _vF_, fore wing; _sch_, halter; _lm_, longitudinal—_b-r_, lateral muscles.—After Graber. ]